U.S. patent number 6,215,682 [Application Number 09/529,030] was granted by the patent office on 2001-04-10 for semiconductor power converter and its applied apparatus.
This patent grant is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Masahiko Akamatsu.
United States Patent |
6,215,682 |
Akamatsu |
April 10, 2001 |
Semiconductor power converter and its applied apparatus
Abstract
In order to keep constant an outlet temperature of a refrigerant
for cooling a semiconductor power device, continuous and variable
control of flow rate of the refrigerant is performed. At the same
time, a variable speed fan is continuously and variably controlled
in response to a temperature control so that an inlet temperature
of the refrigerant is kept constant.
Inventors: |
Akamatsu; Masahiko (Tokyo,
JP) |
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha (Tokyo, JP)
|
Family
ID: |
14209033 |
Appl.
No.: |
09/529,030 |
Filed: |
April 6, 2000 |
PCT
Filed: |
September 18, 1998 |
PCT No.: |
PCT/JP98/04225 |
371
Date: |
April 06, 2000 |
102(e)
Date: |
April 06, 2000 |
PCT
Pub. No.: |
WO00/17927 |
PCT
Pub. Date: |
March 30, 2000 |
Current U.S.
Class: |
363/141;
257/E23.098; 361/699 |
Current CPC
Class: |
H01L
23/473 (20130101); H01L 2924/3011 (20130101); H01L
2924/0002 (20130101); H01L 2924/0002 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
H01L
23/34 (20060101); H01L 23/473 (20060101); H05K
007/20 () |
Field of
Search: |
;363/141 ;361/699 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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58-186956 |
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Nov 1983 |
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JP |
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63-192254 |
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Aug 1988 |
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JP |
|
649817 |
|
Feb 1989 |
|
JP |
|
2120690 |
|
May 1990 |
|
JP |
|
2208479 |
|
Aug 1990 |
|
JP |
|
4130698 |
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May 1992 |
|
JP |
|
5168238 |
|
Jul 1993 |
|
JP |
|
6233544 |
|
Aug 1994 |
|
JP |
|
Primary Examiner: Berhang; Adolf Deneke
Attorney, Agent or Firm: Leydig, Voit & Mayer, Ltd.
Claims
What is claimed is:
1. A semiconductor power converter for power conversion between an
input power system and an output power system by controlling
current with a power semiconductor device, comprising:
a cooling liquid path for flow of a cooling liquid to cool a power
semiconductor device which generates heat;
liquid delivery means for delivering the cooling liquid through the
cooling liquid path;
electricity detecting means for detecting operating voltage and
operating current of the semiconductor power converter to determine
heat loss of the semiconductor power converter; and
flow control means for controlling flow rate of the cooling liquid
via the liquid delivery means and providing continuous and variable
control of the flow rate of the cooling liquid in response to the
heat loss determined by the electricity detecting means.
2. The semiconductor power converter according to claim 1, further
comprising temperature difference detecting means for detecting a
difference between temperature of the cooling liquid prior to
cooling of the power semiconductor device and after cooling of the
power semiconductor device wherein the flow control means controls
the flow rate of the cooling liquid in response to the temperature
difference detected by the temperature difference detecting
means.
3. The semiconductor power converter according to claim 1, wherein
the electricity detecting means issues a command based on the heat
loss of the semiconductor power converter.
4. The semiconductor power converter according to claim 1, wherein
the output power system is an AC motor and the operating voltage
and operating current are associated with torque of the AC
motor.
5. The semiconductor power converter according to claim 1, wherein
the flow control means includes storage means for storing emergency
cooling liquid, valve means for delivering to the cooling liquid
path the emergency cooling liquid stored in the storage means,
signal generating means for generating a signal when the operating
current flowing through the power semiconductor device exceeds a
threshold level, and an emergency cooling command circuit for
opening the valve means in response to the signal generated by the
signal generating means.
6. The semiconductor power converter according to claim 5, further
comprising:
a cooling apparatus for cooling the emergency cooling liquid to a
temperature lower than a temperature of the cooling liquid prior to
cooling of the power semiconductor device.
7. The semiconductor power converter according to claim 5, wherein
the flow rate of the cooling liquid is increased by the flow
control means in response to the signal of the signal generating
means.
8. An apparatus including the power semiconductor converter of
claim 5, and including a DC power transmission system for
connecting a plurality of AC power systems by DC lines, wherein,
upon detection of a ground fault on one of the DC lines, conversion
of the semiconductor power converter at a first pole side close to
the ground fault is stopped and an output of the semiconductor
power converter at a second pole side is increased and the
emergency cooling liquid is delivered by controlling the valve
means of the semiconductor power converter at the second pole side.
Description
TECHNICAL FIELD
The present invention relates to improvement of a semiconductor
power converter employing a power semiconductor device, and its
applied apparatus.
BACKGROUND ART
Semiconductor power converters employing power semiconductor
devices have been widely used so far. Since heat release of the
power semiconductor device may cause malfunction or fracture of the
semiconductor power converter, the semiconductor power converter is
used by cooling the power semiconductor device. In order to cool
the power semiconductor device, a forced cooling method is usually
adopted in which one of gaseous and liquid cooling fluids such as
air, water, fluorocarbon and insulating oil is caused to flow
according to scale of the semiconductor power converter and
magnitude of heat release value.
This cooling method has been adopted to such an extent that cooling
fluid is caused to flow at a predetermined constant quantity or
rate determined for a maximum heat release value of the
semiconductor device by capability of a fan, a pump, a cooler or
the like in order to simplify structure and upgrade economy of the
semiconductor power converter as much as possible or that even if
control is performed, on-off control of cooling power at the utmost
is performed for the purpose of saving the cooling power.
For example, an arrangement of a cooling apparatus of a
semiconductor power converter disclosed in Japanese Patent
Laid-Open Publication No. 4-130698 (1992) is shown in FIG. 13. In
order to explain operational characteristics of the cooling
apparatus, FIG. 14 shows temperature changes of a semiconductor
device. In FIG. 13, a multiplier 201 multiplies a detected current
of the semiconductor power converter (not shown) by an intrinsic
constant of the semiconductor power converter so as to obtain a
value corresponding to heat release of the semiconductor device. An
integrator 202 integrates with respect to time a value obtained by
subtracting the value corresponding to heat release of the
semiconductor device from the multiplier 201, from a set heat
dissipation constant of a setter 203 so as to obtain a value
corresponding to a temperature of the semiconductor device. A
comparator 204 compares a set temperature of a setter 205 and an
output of the integrator 202 with each other so as to obtain a
control output for effecting changeover between operation and stop
of a cooling fan (not shown).
FIG. 14 shows that the semiconductor power converter is turned on
and off alternately several times before temperature of the
semiconductor device reaches a set value and the cooling fan is
operated for the first time at a time t0. Namely, useless cooling
power during this period is saved. In short, since the cooling fan
is operated only when there is a risk that the semiconductor device
may be fractured thermally, useless operation of the cooling fan is
lessened.
However, in the above described cooling method in which cooling
capability is kept constant or is subjected to on-off control, it
is inevitable that temperature of the semiconductor device
fluctuates due to variations of output of the semiconductor device
as shown in FIG. 14 or although not specifically shown in FIG. 14,
temperature variations of the semiconductor device (referred to
also as a "heat cycle", hereinafter) occur in response to turning
on and off of the cooling fan. In some cases, the heat cycle
adversely affects the semiconductor device more than operating
temperature so as to deteriorate the semiconductor device and
reduce service life of the semiconductor device, thereby resulting
in rise of its failure rate.
Especially, in case a load is a variable speed motor in application
of the power converter, such a problem arises that since
acceleration and deceleration of the variable speed motor, i.e.,
increase and reduction of current to the semiconductor device are
repeated frequently, so that a severe heat cycle is applied to the
semiconductor device even in a constant state of the cooling fan
and thus, temperature of the semiconductor device fluctuates
frequently, thereby resulting in drop of reliability of the power
converter.
The conventional cooling method in which cooling capability is kept
constant or is subjected to on-off control has the disadvantage
that temperature of the semiconductor device undergoes the heat
cycle as described above but has another drawback as follows. In
most cases, capability for cooling the semiconductor device is set
at a constant value in accordance with a maximum output of the
semiconductor power converter. The maximum output referred to above
represents one yielded for a period of not less than dozens of
seconds (referred to as a "rated output") and heat release due to
overcurrent flowing for a duration shorter than the above period is
not cooled. This is because if cooling capability controlled to be
constant is raised in accordance with the short-time overload
rating, cooling capability to be seldom used should be secured at
all times, which is quite wasteful.
As a result, for short-time overcurrent, rise of temperature of the
semiconductor device is determined by thermal capacity of the
semiconductor device itself and thermal capacity of a heat sink
(referred to also as a "heat dissipation fin" or a "heat
exchanger") to which the semiconductor device is attached.
However, for the purpose of raising reliability of the system,
there is always a demand for setting short-time overcurrent
capability of the power converter to as high a level as possible.
On the other hand, the short-time thermal capacity of the heat sink
is restricted by heat transfer rate of material of the heat sink.
Therefore, such a problem is posed that a limit is reached in
raising the short-time overcurrent capability of the power
converter in comparison with the continuous rating.
As one example in which the above mentioned problem affects a
concrete configuration of the system, a case of an applied
apparatus in which a semiconductor power converter is applied to a
DC power transmission system is described. In case a DC power
transmission system, for example, is used by connecting a plurality
of converters to the system and a fault happens in which rise of
fault current is slightly gentle, i.e., for several seconds as in a
ground fault of power transmission lines in the DC system, only a
power transmission line associated with the ground fault is
disconnected by using a high-speed circuit breaker and the circuit
breaker is reclosed after recovery of the ground fault. This is
described with reference to FIG. 15 showing an arrangement of a
conventional DC power transmission system. FIG. 15 illustrates a
typical DC power transmission system similar to that described in a
book entitled "Electrical Engineering Handbook" edited by the
Institute of Electrical Engineers of Japan. In FIG. 15, "1a" and
"2b" denote different AC power systems, "2a" to "2d" denote
converter transformers, "Da" and "Db" denote power converters used
exclusively for rectification and formed by diodes, "3c" and "3d"
denote separately excited power converters for inverters, which are
formed by semiconductor devices, "Cba" to "CBh" denote DC circuit
breakers, "6a" and "6b" denote neutral grounds and "7a" to "7d"
denote DC power transmission lines.
Ordinary operation of the DC power transmission system is performed
in a state where the DC circuit breakers CBa to CBh are closed. AC
power supplied from the AC system 1a via the converter transformer
2a is rectified by the power converter Da used exclusively for
rectification and is transmitted as DC power by the DC power
transmission lines 7a to 7d. Then, the DC power is again converted
to AC by the separately excited power converter 3c and is
transmitted to the AC power system 1b through the transformer 2c.
In the foregoing, only an upper half portion of FIG. 15 is
described but the same applies to a lower half portion of FIG. 15.
If a ground fault LG occurs in the DC power transmission line 7b,
both of the DC circuit breakers CBb and CBf are interrupted
promptly before its current fractures the power converter. Thus,
the current of the DC power transmission line 7b is commutated to
the DC power transmission line 7a temporarily and the DC circuit
breakers are reclosed upon recovery of the ground fault so as to
reinstate the DC power transmission system to ordinary
operation.
By operating the DC power transmission system as described above,
period during which transmission power drops is minimized. However,
since it is extremely difficult to manufacture a extra-high voltage
DC circuit breaker designed for the purpose of high-speed
interruption, such disadvantages are incurred that the system
becomes expensive and transmission voltage cannot be set high.
As a method of restraining ground fault current without using the
DC circuit breaker, impedance grounding (capacitor grounding) is,
needless to say, is known. Detailed description of this method is
abbreviated here. When a ground fault occurs in case this method is
adopted, voltage to ground at a side of a power transmission line,
which is not subjected to the ground fault rises to twice an
ordinary value, so that dielectric strength to ground of the DC
line should be set at twice that required for ordinary power
transmission. Therefore, since such an essential merit is lost that
cost for constructing a power transmission line for DC power
transmission is more inexpensive than that for AC power
transmission, this method is seldom used for extra-high voltage DC
power transmission.
As described above, the conventional semiconductor power converters
have the drawbacks that the power semiconductor device is readily
subjected to the heat cycle and the short-time overcurrent
capability of the power semiconductor device is not so high as to
be satisfactory.
Meanwhile, in the case of an applied apparatus in which the
conventional semiconductor power converter is applied to a DC power
transmission system, such an inconvenience is incurred that in
order to prevent drop of electric energy to be transmitted in the
system at the time of a ground fault, the high-speed circuit
breakers for two circuits and the power transmission lines for two
circuits are required to be provided, which is expensive.
DISCLOSURE OF INVENTION
A semiconductor power converter for performing power conversion
between an input power system and an output power system by
controlling current by the use of a power semiconductor device,
according to the present invention comprises: a cooling liquid path
for cooling with cooling liquid the power semiconductor device
which generates heat; a liquid delivery means for delivering the
cooling liquid through the cooling liquid path; a liquid
temperature detecting means for detecting a temperature of the
cooling liquid after cooling of the power semiconductor device; and
a flow control means for controlling a flow rate of the cooling
liquid via the liquid delivery means; wherein continuous and
variable control of the flow rate of the cooling liquid is
performed by the flow control means in accordance with the
temperature of the cooling liquid detected by the liquid
temperature detecting means.
Meanwhile, a temperature difference detecting means for detecting a
difference between a temperature of the cooling liquid prior to
cooling of the power semiconductor device and that after cooling of
the power semiconductor device is further provided such that the
flow rate of the cooling liquid is controlled by the flow control
means in accordance with the temperature difference detected by the
temperature difference detecting means.
Meanwhile, the liquid temperature detecting means is replaced by an
electricity detecting means for detecting a quantity of electricity
corresponding to heat release of the semiconductor power converter
such that the flow rate of the cooling liquid is controlled by the
flow control means in accordance with a detection output of the
electricity detecting means.
Meanwhile, the electricity detecting means is replaced by a command
means for issuing a command on the quantity of electricity
corresponding to the heat release of the semiconductor power
converter.
Meanwhile, the liquid temperature detecting means is replaced by a
device temperature detecting means for detecting a temperature of
the power semiconductor device.
Meanwhile, the device temperature detecting means calculates a heat
release value or the temperature of the power semiconductor device
from hysteresis of the current of the power semiconductor
device.
Meanwhile, the output power system is an AC motor and the quantity
of electricity is a quantity associated with a torque of the AC
motor.
The flow control means includes a storage means for storing
emergency cooling liquid, a valve means for delivering to the
cooling liquid path the emergency cooling liquid stored in the
storage means, a signal generating means for generating a signal
when the current flowing through the power semiconductor device
exceeds a predetermined level and an emergency cooling command
circuit for opening the valve means in response to the signal of
the signal generating means.
Meanwhile, a cooling apparatus for cooling the emergency cooling
liquid to a temperature lower than a temperature of the cooling
liquid prior to cooling of the power semiconductor device is
further provided.
Meanwhile, the flow rate of the cooling liquid is increased by the
flow control means in response to the signal of the signal
generating means.
Meanwhile, in an applied apparatus of the power semiconductor power
converter, a DC power transmission system for connecting a
plurality of AC power systems by DC lines of positive and negative
poles is formed and upon detection of a ground fault on one of the
DC lines of positive and negative poles, conversion of the
semiconductor power converter at one pole side close to the ground
fault is stopped and an output of the semiconductor power converter
at the other normal pole side is increased and the emergency
cooling liquid is delivered by controlling the valve means of the
semiconductor power converter at the normal pole side.
Meanwhile, a semiconductor power converter for performing power
conversion between an input power system and an output power system
by controlling current by the use of a power semiconductor device,
comprises: an air cooling means for cooling with air the power
semiconductor device which generates heat; an air velocity control
means for performing continuous and variable control of an air
velocity of the air cooling means; and at least one of (1) an
electricity detecting means for detecting a quantity of electricity
corresponding to heat release of the power semiconductor device,
(2) a command means for issuing a command on the quantity of
electricity corresponding to the heat release of the power
semiconductor device and (3) a temperature detecting means for
detecting a temperature of the power semiconductor device; wherein
continuous and variable control of the air velocity is performed by
the air velocity control means in accordance with an output of one
of the (1), (2) and (3).
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a view showing an arrangement of a semiconductor power
converter according to a first embodiment of the present
invention.
FIG. 2a is a fragmentary detail view of FIG. 1 and FIG. 2b is a
graph of temperature and fan speed characteristics of the circuit
of FIG. 2a.
FIG. 3 is a view showing an arrangement of a semiconductor power
converter according to a second embodiment of the present
invention.
FIG. 4 is a fragmentary detail view of FIG. 3.
FIG. 5 is a view showing an arrangement of a semiconductor power
converter according to a third embodiment of the present
invention.
FIG. 6 is a view showing an arrangement in which a portion of FIG.
5 is modified.
FIG. 7 is a view showing an arrangement of a semiconductor power
converter according to a fourth embodiment of the present
invention.
FIG. 8 is a view explanatory of characteristics of FIG. 7.
FIG. 9 is a view showing an arrangement of a semiconductor power
converter according to a fifth embodiment of the present
invention.
FIG. 10 is a view showing an arrangement of a semiconductor power
converter according to a sixth embodiment of the present
invention.
FIG. 11 is a view showing a DC power transmission system according
to a seventh embodiment of the present invention.
FIG. 12 is a detail view of FIG. 11.
FIG. 13 is a view explanatory of a cooling system for a
conventional semiconductor power converter.
FIG. 14 is a view explanatory of characteristics of FIG. 13.
FIG. 15 is a view showing a conventional DC power transmission
system.
BEST MODE FOR CARRYING OUT THE INVENTION
(First Embodiment)
A semiconductor power converter according to a first embodiment of
the present invention is shown in FIG. 1. In FIG. 1, "3" is a main
portion of the semiconductor power converter, "301" is a power
semiconductor device referred to also as a "semiconductor stack"
and including a path for delivering cooling liquid, "10" is a
liquid path (pipe) for carrying the cooling liquid referred to also
as "refrigerant" below and "18" is a refrigerant supply system for
the semiconductor stack 301. In FIG. 1, the refrigerant supply
system 18 supplies the cooling liquid by way of the pipe 10. "19"
is a heat exchanger for maintaining quality of the refrigerant and
performing heat exchange. The refrigerant flows in the pipe 10 in
the direction of the arrow. "20a" is a sensor (temperature
detecting means) for detecting temperature of the refrigerant at an
inlet side of the stack 301, namely, prior to cooling of the stack
301, while "20b" is a sensor (temperature detecting means) for
detecting temperature of the refrigerant at an outlet side of the
stack 301, namely, after cooling of the stack 301. "193a" is a
variable speed fan for cooling the refrigerant in the heat
exchanger and "193b" is a temperature controller which controls the
variable speed fan 193aso as to keep a detection temperature T1 of
the temperature sensor 20a constant. "21" is a refrigerant flow
control pump (liquid delivery means), "22" is a variable-speed
drive (flow control means) for the refrigerant flow control pump 21
and "23a" is a check valve.
Then, its operation is described with reference to FIG. 1. The
refrigerant is fed from the heat exchanger 19 to an inlet IN of the
stack 301 of the semiconductor power converter 3 by the refrigerant
flow control pump 21 and the refrigerant which has absorbed heat of
the stack 301 is discharged from its outlet OUT. Then, the
refrigerant is returned to the heat exchanger 19 and heat of the
refrigerant is released to external air by the variable speed fan
193a. Meanwhile, an ion exchanger for removing impurities in a
refrigerant tank or the refrigerant may be provided in the course
of the pipe 10 but its illustration and description are abbreviated
due to its less relevance to the present invention.
The variable-speed drive 22 controls flow rate of the refrigerant
flow control pump 21 continuously such that a temperature T2 of the
refrigerant (a detection temperature of the temperature detecting
means 20b) after cooling of the stack 301 assumes a predetermined
value. Namely, the variable-speed drive 22 performs stepless
variable speed control so as to accelerate and decelerate rotation
of the pump 21 in accordance with the temperature T2 when the
temperature T2 is higher than and lower than the predetermined
temperature, respectively. A control configuration and
characteristics of the variable speed control unit 22 are shown in
FIGS. 2a and 2b, respectively. "22a" in FIG. 2 is an arithmetic
amplifier. At this time, as speed change rate of the pump 21
relative to temperature error, (gain gradient), becomes higher as
shown by the line (2) instead of the line (1) in FIG. 2b, its
temperature control accuracy is higher.
Speed of the variable speed fan 193a of the heat exchanger 19 is
controlled so as to make the temperature T1 of the refrigerant
substantially constant. If heat dissipation capacity of the heat
exchanger 19 is sufficient and the temperature T1 of the
refrigerant at an outlet of the heat exchanger 19 is stable,
temperature of the stack 301, therefore, a heat cycle of device
temperature of the semiconductor device is lessened greatly by the
above mentioned constant control of the temperature T2.
In FIG. 1, the inlet temperature T1 of the refrigerant is detected
by the temperature sensor 20a and temperature of the refrigerant is
controlled to T1 by the temperature controller 193b and the
variable speed fan 193a. However, if the variable speed fan has a
sufficient cooling capability even at a constant rotational speed,
these devices are not required to be provided necessarily.
(Second Embodiment)
In case capability of the heat exchanger 19 is not sufficient and
variations of the temperature T1 of the refrigerant prior to
cooling of the stack 301 are great, such a phenomenon may happen in
which setting of flow rate of the refrigerant at a maximum value
does not lead to drop of the temperature T2 through the constant
control of the temperature T2 performed in the first embodiment.
Therefore, in such a case, the inlet temperature T1 and the outlet
temperature T2 of the refrigerant may be detected such that flow
rate of the refrigerant is controlled in accordance with a
difference between the inlet temperature T1 and the outlet
temperature T2, i.e., .DELTA.T=T2-T1 as shown in FIGS. 3 and 4.
Thus, since burden of a heat cycle is lessened further, such an
effect can be gained that its reliability and service life are
improved further.
(Third Embodiment)
In order to lessen burden of a heat cycle of the semiconductor
device, constant control of device temperature or temperature of a
junction of the device is more effective than constant temperature
control of a heat sink.
Loss of the power semiconductor device depends on operating current
I and operating voltage V. Namely, momentary voltage and momentary
current are monitored and the loss L can be calculated based on the
momentary voltage and the momentary current. In order to gain
effect of the present invention, approximate calculation is
sufficient for calculating the loss. Therefore, the loss can be
calculated from either a calculation result table prepared
preliminarily for respective operational modes in view of switching
conditions on the basis of the power source voltage V of the power
converter and the converter output current I or a known approximate
calculation formula.
Real-time information required for performing these processings can
be easily obtained from the semiconductor power converter 3. In
FIG. 5, "31" denotes a calculation means for calculating
correlative amount of rise of temperature of the semiconductor
junction. FIG. 5 illustrates that a signal Y corresponding to rise
of the temperature of the junction is obtained from the thus
obtained loss L by the calculation means 31 and flow rate of the
refrigerant is controlled in accordance with this signal Y, for
example, the number of revolutions of the pump 21 is controlled in
proportion to the signal Y.
A configuration which is made with higher precision than that of
FIG. 5 is shown in FIG. 6. In FIG. 6, a calculation means 32 for a
temperature Tj of the semiconductor junction or its rise .DELTA.Tj
performs estimative calculation of the temperature Tj of the
semiconductor junction from the outlet temperature T2 or the inlet
temperature T1 of the refrigerant and the amounts V, I and L
associated with heat release value. Then, the variable speed
control unit 22 controls flow rate of the refrigerant such that the
temperature Tj or the rise .DELTA.Tj obtained by the above
described calculation follows a command value tj or .DELTA.tj.
By the arrangements of FIGS. 5 and 6, since temperatures of main
portions such as the junction, a wafer and a cathode face in which
the heat cycle is problematical can be controlled more directly and
with higher precision, burden of the heat cycle is lessened
further, namely, temperature variations are reduced, so. that such
an effect can be achieved that its reliability and service life are
improved further.
(Fourth Embodiment)
In the first to third embodiments, the heat cycle is relatively
gentle in the case where temperature changes of the power
semiconductor device are caused by normal load variations or
variations of air temperature with time. However, changes of output
current of the semiconductor power converter 3 are not necessarily
so gentle as those of the first to third embodiments. For example,
in case the semiconductor power converter 3 is associated with a
power system such as a DC power transmission system, the
semiconductor power converter should be burdened with whole of DC
power transmission capability including fault current, for a short
period of several to dozens of cycles so as to cope with an
emergency, namely, until removal of a fault portion is completed in
the case of a ground fault as described earlier with respect to
prior art.
In such short-time overload, overcurrent capability of the
semiconductor device relative to overload current falling in a time
region of the above mentioned ground fault or the like can be
raised by rapidly cooling the power semiconductor device 3.
In FIG. 7, a system for rapidly cooling the semiconductor device is
provided for the above mentioned purpose, namely, for the purpose
of raising short-time overload capacity. In FIG. 7, "26" is an
emergency refrigerant storage tank (emergency refrigerant storage
means), "23b" is a check valve, "24" is an auxiliary refrigerant
pump, "25" is a drive control unit for the auxiliary refrigerant
pump 24, "27" is a detector for detecting storage pressure or
storage amount in the emergency refrigerant storage tank 26, "28"
is an emergency cooling liquid delivery valve and "29" is a control
mechanism for the valve 28, which acts also as an emergency cooling
command circuit.
Meanwhile, "30" is a metabolic refrigerant bypass capillary for
causing a small amount of the refrigerant to successively flow
therethrough so as to prevent change of properties of the
refrigerant in the storage tank 26 due to residence of the
refrigerant in the storage tank 26. The valve 28 may also be
slightly opened at all times in place of the bypass capillary 30.
In FIG. 7, when the short-time overload capacity of the
semiconductor power converter 3 is required to be raised as
described above, namely, the ground fault referred to above occurs
in DC power transmission, a fault signal X is outputted from a
signal generating means (not shown) and is used here as an
emergency cooling command.
Then, its operation is described. If a load is applied to the
semiconductor power converter 3 or an overload such as a ground
fault is applied to a power transmission line (or an overload
command is given to the semiconductor power converter 3), the
signal X is outputted from the signal generating means (not shown).
In response to the signal X, the valve control mechanism (emergency
cooling command circuit) 29 outputs the emergency cooling command
so as to open the emergency cooling liquid delivery valve 28 such
that the refrigerant stored in the refrigerant storage tank 26 is
emergently released to the stack 301, thereby resulting in sharp
rise of cooling capability.
As a result, since short-time overload capacity of the
semiconductor can be raised temporarily, rise of its temperature
can be restrained. The refrigerant storage tank 26 includes a
piston (not shown) or the like depressed by inert gas or a pressure
spring and the refrigerant is introduced into the tank 26 under
pressure by the pump 24 such that the tank 26 is kept under
pressure at all times. By this pressure, the refrigerant is
intensely released to the stack 301 concurrently with opening of
the valve 28. When pressure in the tank 26 drops subsequently, the
pressure detector 27 detects the pressure drop and the pump 24 is
started by this signal of the detector 27 such that the refrigerant
is stationarily supplied at a high flow rate continuously.
Therefore, the refrigerant in an amount corresponding to a short
period required for priming the pump 24 may be stored in the
refrigerant storage pump 26. Since an emergency happens only
rarely, deterioration of the refrigerant may advance if the
refrigerant is left in the tank for a long term. Therefore, a small
amount of the refrigerant is discharged on purpose by way of the
capillary 30. Furthermore, the check valve 23a is provided for
preventing back flow of the refrigerant towards the refrigerant
flow control pump 21 at the time the emergency refrigerant delivery
valve 28 has been opened. The check valve 23b is provided also at
the refrigerant storage tank 26 so as to maintain pressure in the
tank 26 even at the time of stop of the pump 24.
Capacity of the refrigerant storage tank 26 is limited. Thus, even
if the tank 26 is pressurized by the pump 24 in response to
discharge of the cooling liquid, pressure in the tank 26 drops in
the meantime. Therefore, flow rate of the cooling liquid, which is
obtained at the time of start of discharge of the cooling liquid,
does not continue indefinitely. Accordingly, there should be a
duration effective for raising the short-time overload capacity in
this method. In order to facilitate understanding of this
conclusion, FIG. 8 illustrates changes of the short-time overload
capacity of the power semiconductor stack 301 due to cooling. In
FIG. 8, "100" represents the short-time overload capacity of the
stack 301 with no cooling, "101" represents the short-time overload
capacity in the case where cooling corresponding to an average
rated power has been performed and "102" represents the short-time
overload capacity in the case where emergency cooling of FIG. 7 is
performed. A duration in which the characteristics 102 prevail is
affected by capacity of the tank 26.
On the other hand, if the tank has such a capacity as to maintain
its flow rate for an acceleration completion period of 0.3 to 1
sec. in the motor pump 24, overload capacity can be maintained for
a long time in accordance with capability of the motor pump 24.
Namely, the tank 26 has an effect of rapidly raising cooling
capability during delay of the acceleration period of the pump.
Furthermore, if the acceleration period is shortened to dozens of
ms. by forming the variable speed motors 24 and 25 by a low-inertia
motor or the like in the above mentioned embodiment and the
following embodiments in order to improve the overload capacity
requiring rapid rise, the tank can also be eliminated.
(Fifth Embodiment)
FIG. 9 shows an arrangement in which the effect of the emergency
cooling method of the fourth embodiment is further heightened. In
FIG. 9 illustrating only a periphery of the refrigerant storage
tank 26 of FIG. 7, other portions are similar to those of FIG. 7
and thus, the illustration is abbreviated. In FIG. 9, "120" is a
cooler for keeping temperature of the refrigerant in the emergency
refrigerant storage tank 26 as low as possible, for example, lower
than the temperature T1 of the refrigerant in the pipe 10 and "121"
is a radiator. Thus, emergency cooling capability is raised
further. As a result, capability to cope with an emergency and the
short-time overload capacity are improved further. In this case, it
is preferable that the tank 26 is a so-called heat insulation tank
having excellent heat insulating properties.
(Sixth Embodiment)
Another embodiment of the present invention is shown in FIG. 10. In
that the refrigerant flow control pump 21 and the auxiliary
emergency pump 24 are connected to each other in series, FIG. 10 is
different from FIG. 7 of the fourth embodiment in which the two
pumps are connected to each other in parallel. Since the pumps are
connected to each other in series, the pumps may be disposed at any
one of locations 21b, 21c, 21d and 21e in addition to the locations
shown in FIG. 10 and thus, it is firstly possible to gain such an
effect that degree of freedom of layout of the pumps is raised.
In case pressure loss of the pipe 10 and the stack 301 is small
when the auxiliary emergency pump 24 is actuated, flow rate of the
refrigerant increases sufficiently even if the pumps 21 and 24 are
connected to each other in parallel. However, if the pressure loss
is large, a predetermined flow rate cannot be obtained if delivery
pressure is not increased in accordance with increase of the flow
rate.
In the fourth embodiment of FIG. 7, since the refrigerant storage
tank 26 supplies this pressure, capacity of the flow rate of the
pump 24 has degree of freedom. On the other hand, since the tanks
21 and 24 are connected to each other in parallel, the check valve
23a is closed when pressure in the tank 26 is high, while the check
valve 23b is closed when pressure in the tank 26 drops. As a
result, there is a risk that the refrigerant is supplied by only
one of the two refrigerant flow paths at all times.
On the other hand, in FIG. 10, the above mentioned pressure loss
may be shared by both of the pumps 21 and 24. Therefore, it is
possible to achieve an effect that flow rate of the refrigerant in
case of an emergency can be increased positively.
(Seventh Embodiment)
A concrete example of application of control of the system by the
command X mentioned in the fourth to sixth embodiments is described
with reference to FIGS. 11 and 12. In FIGS. 11 and 12, "3a to 3d"
are semiconductor power converters, "4a to 4d" are DC capacitors,
"6a and 6b" are ground, "7a and 7b" are positive and negative power
transmission lines, "18a to 18c" are cooling liquid supply systems
corresponding to the semiconductor power converters 3a to 3d, "10"
is a refrigerant pipe, "11a to 11d" are circuit breakers at an AC
side of the semiconductor power converters and "12a to 12d" are
command means for outputting emergency cooling commands Xa to
Xd.
"302" is a switching device in the semiconductor power converter,
"303" is a reverse-current carrying device (diode) and "13 to 17"
are current detecting sensors (current detecting means) inserted at
the illustrated locations. Meanwhile, "18a to 18d" include, for
example, the emergency cooling system shown in one of FIGS. 7 to
10.
Then, its operation is described. In case a ground fault LG occurs
on the DC line 7a, overcurrent flows through the semiconductor
power converter 3a and the DC capacitor 4a, the power transmission
line 7a, the ground fault point LG and the current detecting means
14 to 17 so as to return to the semiconductor power converter 3a
and the DC capacitor 4a at the original location. At the same time,
overcurrent also flows through the current detecting means 13
provided on the AC line of the converter 3a.
Likewise, overcurrent flows through the semiconductor power
converter 3c and the DC capacitor 4c, the power transmission line
7a, the ground fault point LG and the current detecting means 14 to
17 so as to return to the semiconductor power converter 3c and the
DC capacitor 4c at the original location. At the same time,
overcurrent also flows through the current detecting means 13
provided on the AC line of the converter 3c.
On the basis of one or a plurality of ones of outputs of the
current detecting means, the command means 12a or 12c outputs the
command X1 or X3 (alternatively, the command means 12a and 12c may
output the commands X1 and X3). Since overcurrent closer to the
ground fault point LG increases more quickly, the command means 12
closer to the ground fault point LG outputs the command X earlier.
Meanwhile, since current of the DC capacitor 4 increases more
quickly than converter current flowing through the AC circuit,
outputs of the current detecting sensors 15, 16 and 17 for
detecting discharge current of the capacitors 4 increase
quickly.
On the other hand, if monitoring reveals that these outputs exceed
a predetermined value, the emergency commands X1 or X3 is outputted
so as to stop operation of the corresponding semiconductor power
converter 3a or 3c by so-called gate interrupting operation.
Simultaneously, the emergency refrigerant output valves 28 of the
cooling liquid supply systems 18b and 18d connected with the
semiconductor power converters 3b and 3d are opened so as to
increase flow rate of the refrigerant. Moreover, in response to the
signals X1 and X3, output currents of the normal semiconductor
power converters 3b and 3d are increased by quantity corresponding
to that of current shouldered by the semiconductor power converters
3a and 3c until then. Meanwhile, at the same time, flow rate of the
pump 21 may also be increased.
By the foregoing arrangement, cooling capability is raised sharply,
so that quantity of power which can be transmitted by the normal
semiconductor power converter can be increased and thus,
transmission power of the whole power transmission system can be
maintained.
If the ground fault current LG of the DC power transmission line 7a
disappears and insulation to the earth is recovered, operation of
all the semiconductor power converters 3 is restarted by reclosing
the circuit breakers 11. The increased transmission power of the
normal poles is naturally recovered to the original level and the
cooling system also returns to its original level sequentially. As
a result, reliability of power supply of the power transmission
system can be upgraded in a wide sense.
In the above description, the normal semiconductor power converters
shoulder a whole of the interrupted current. However, it is
needless to say that even if, for example, one-third or two-thirds
of the interrupted current is shouldered, the effects can be gained
to some extent.
Also in an AC system, a total transmission power may decrease to
its two-thirds as in a fault of short-time one-phase opening in
three-phase AC power transmission. Therefore, in order to secure
two-thirds of the transmission power by only the normal poles, the
following relation is obtained.
Namely, if the power converters of the one side are capable of
performing short-time overload power transmission which is 4/3
times that of ordinary operation, reliability equivalent to that of
the AC power transmission system can be obtained in DC power
transmission. At this time, if device loss is approximated to be
proportional to square of the current, cooling capability may be
increased to about 16/9 times. Assuming that cooling capability
should be increased twice in view of its allowance, the above
calculative target can be fully achieved when not only flow rate of
the refrigerant is set twice but temperature of the refrigerant is
lowered.
In order to secure a quantity of power transmitted at the time of a
ground fault in a prior art DC power transmission system shown in
FIG. 15, a power transmission line of the ground fault is required
to be disconnected by a DC circuit breaker. However, at the time of
a ground fault in FIGS. 10 and 11, since transmission power and
current can be shouldered by the normal converters by intensifying
cooling, operation of the power converters connected to a power
transmission line of the ground fault may be stopped and a circuit
breaker at an AC side may be disconnected. Therefore, the expensive
DC circuit breaker and spare power transmission lines become
unnecessary.
In the foregoing description, the fault of the power system, which
is directly associated with the converters 3, has been described.
In this emergent state, such a case may happen that overload
operation should be performed by other requirements or requirements
of other converters. In this case, the emergency command X is
issued not upon detection of overcurrent but as increase of an
overload operation command or a command on output of the
converters. Therefore, it is also possible to employ an arrangement
in which in response to the command X from another converter (not
shown), cooling capability of still another converter is
controlled.
Meanwhile, in all the above embodiments and examples, the
refrigerant has been described as liquid. In the case of liquid
cooling, since its thermal capacity is larger than that of air, the
effects of the present invention become more conspicuous than air.
However, the present invention may also be applicable to air.
In the case of air cooling, only if a measure for reducing heat
transfer resistance is taken, for example, the number of heat
dissipation fins of the semiconductor stack is increased or maximum
wind velocity is raised, the effects of the present invention can
be gained in the same manner as liquid cooling.
In the semiconductor power converter of the present invention,
since control is performed such that temperature of the refrigerant
after cooling of the device is kept constant or difference between
temperature of the refrigerant prior to cooling of the device and
that after cooling of the device is kept constant as described
above, the heat cycle to which the device is subjected is lessened,
so that such effects are achieved that service life and reliability
of the device can be upgraded.
Meanwhile, since the refrigerant kept at especially low temperature
is intensely fed when overcurrent flows through the device or
overload operation is performed, the effect that the short-time
overcurrent capability (short-time burden) can be raised is
obtained.
In the case of the power converter for DC power transmission, since
the normal converters may be burdened with current at the time of
the ground fault by intensifying cooling, operation of the power
converters connected to the power transmission line of the ground
fault may be stopped by disconnecting the circuit breaker at the AC
side, so that such effects are achieved that the expensive DC
circuit breaker and spare power transmission lines become
unnecessary. Meanwhile, when there is a need to perform overload
power transmission by another external overload, the effect is
obtained that this rapid power transmission capability is
upgraded.
Industrial Applicability
The semiconductor power converter of the present invention can be
used not only as a converter of a power system such as a frequency
converter and an inverter for solar thermal power generation and
windmill power generation but as semiconductor power converters of
all applications such as railroad, general industry and
watercraft.
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